1. Field of the Invention
This invention relates to a control system for a chassis dynamometer.
2. Description of the Prior Art
The control system for the chassis dynamometer includes a torque control system in combination of a feedforward control and a feedback control.
FIG. 1 is a block diagram of a prior art control system for a chassis dynamometer applying the torque control system.
In FIG. 1, element 1 is a roller for carrying a drive wheel of a vehicle to be tested; element 2 is a dynamometer provided with a generator serving as a driving force absorption device; element 3 is a shaft directly connecting the roller 1 and dynamometer 2; elements 4 and 5 are designate a torque sensor and a speed sensor mounted to the shaft 3 respectively, and element 6 is an electrical power converter which controls a field current or an exciting current to the dynamometer on the basis of a control signal from a control system 10 to be discussed below so as to thereby allow the dynamometer to absorb a force output from a vehicle.
The control system 10 comprises a feedforward control circuit 11 carrying out the feedforward control on the basis of measured values by the torque sensor 4 and speed sensor 5 and a PI (Proportional Integration) control circuit 13 for stably compensating an output of an error function computed in an error function generator circuit 12 and performs feedback control so that the integrated value of an error .epsilon.(t) between a predicted output value F'.sub.PAU (t+.DELTA.T) of the dynamometer 2 and a desired value F.sub.TT (t)+Lm (V) becomes zero.
In other words, when the running resistance as the function of vehicle speed V is represented by RL (V), and the vehicle's weight by I, and the acceleration of vehicle by .alpha., a force F.sub.veh output by the vehicle is given as: EQU F.sub.veh =RL (V)+I.multidot..alpha. (1)
This equation, when represented by the weight Im of a flywheel 7 and an electrical inertia Ie, is given as: EQU F.sub.veh =RL (V)+Im.multidot..alpha.+Ie.multidot..alpha. (2)
Since Im.multidot..alpha. is absorbed by the flywheel 7, a force F.sub.PAU to be absorbed by the dynamometer 2 in the next time step t+.DELTA.T is given by the equation: EQU F.sub.PAU (t+.DELTA.T)=RL (V)+Ie.multidot..alpha. (3)
Here, when F.sub.veh is expressed by use of an output F.sub.TT of torque sensor 4, the following equation should hold: ##EQU1## where Lm (V): mechanical loss.
On the other hand, when a weight of a roller at a motor in the dynamometer 2 is represented by Ir, the following equation holds: EQU I=Im+Ir+Ie (5)
The force F.sub.PAU to be absorbed by the dynamometer 2 is obtained from the equations (1), (4) and (5) as follows: ##EQU2##
The force F.sub.PAU is controlled by the feedforward control circuit (11) and the integrated value of error .epsilon.(t) between the predicted output value F'.sub.PAU (t+.DELTA.T) and the desired value F.sub.TT (t)+Lm (V), which is given by: ##EQU3## is used as the error function so that the feedback control is carried out until the above error function becomes zero.
The control system combines the quick response, a merit of feedforward control and the stability, that of feedback control, which is defective in the following matters. Especially for a lightweight vehicle, instability may be introduced in the control system so as to thereby reduce the stability and thereby become uncontrollable. Also, the PI controller requires two kinds of parameters, whereby a problem has been created in that control adjustment becomes troublesome. Furthermore, when this system is intended to obtain a quick response, overshoot is not avoidable, thereby creating a problem in that a capacity of a capacitor of the power converter 6 should be raised.